1. Field of the Invention
The present invention relates to a control system for controlling a hybrid vehicle having an internal combustion engine and an electric motor as separate propulsion sources, and more particularly to a control system for controlling a hybrid vehicle in regenerating electric energy with an electric motor.
2. Description of the Related Art
There have heretofore been known hybrid vehicles each having an internal combustion engine and an electric motor as separate propulsion sources. There has also been proposed a control system for controlling a hybrid vehicle to increase the regenerative efficiency of an electric motor when it operates in a regenerative mode at the time the hybrid vehicle is decelerating. For example, a control system disclosed in Japanese laid-open patent publication No. 8-112190 fully opens an electrically controlled throttle valve when the electric motor operates in the regenerative mode at the time the hybrid vehicle is decelerating. Accordingly, a mechanical energy loss produced due to pumping losses of the engine when the throttle valve remains closed is reduced, and the kinetic energy of the hybrid vehicle can efficiently be recovered as regenerated energy.
Japanese laid-open patent publication No. 9-135502 discloses a control system for controlling a hybrid vehicle to reduce braking torques applied by engine braking depending on the magnitude of a regenerative braking torque produced by an electric motor when the electric motor operates in a regenerative mode.
The disclosed control system calculates braking torques applied by engine braking when a throttle valve is fully closed and opened, corresponding to the rotational speed of a drive axle at the time the fuel of supply to the engine is stopped while the hybrid vehicle is decelerating. Then, the control system adjusts the regenerative braking torque produced by the electric motor to the range of the calculated braking torques, and operates a throttle valve in an intake passage of the engine in an opening direction depending on the magnitude of the regenerative braking torque produced by the electric motor for thereby reducing the braking torque applied by engine braking. In this manner, an uncomfortable feeling that the driver of the hybrid vehicle has due to a deceleration variation while the hybrid vehicle is running with engine braking applied, and kinetic energy of the hybrid vehicle which has been wasted is recovered as electric energy for better regenerative efficiency.
With the above conventional control system, however, the amount of regenerated energy is limited to the range of the difference between the braking torques produced when the throttle valve is fully closed and opened, and intake air excessively cools the catalytic converter in an exhaust system of the engine because the throttle valve is fully opened while the electric motor is operating in the regenerative mode, with the result that emission characteristics will become impaired when the hybrid vehicle returns from the regenerative mode to an ordinary running mode. Consequently, the control system needs to reduce the regenerative torque generated by the electric motor and to close the throttle valve or to fully close the throttle valve to interrupt the regenerative mode of the electric motor, when the detected temperature of the catalytic converter reaches a predetermined value.